Solid state white light emitter and display using same

ABSTRACT

A light emitting assembly comprising a solid state device coupleable with a power supply constructed and arranged to power the solid state device to emit from the solid state device a first, relatively shorter wavelength radiation, and a down-converting luminophoric medium arranged in receiving relationship to said first, relatively shorter wavelength radiation, and which in exposure to said first, relatively shorter wavelength radiation, is excited to responsively emit second, relatively longer wavelength radiation. In a specific embodiment, monochromatic blue or UV light output from a light-emitting diode is down-converted to white light by packaging the diode with fluorescent organic and/or inorganic fluorescers and phosphors in a polymeric matrix.

FIELD OF THE INVENTION

This invention relates to solid state light emitting devices such aslight emitting diodes and more particularly to such devices whichproduce white light.

BACKGROUND OF THE INVENTION

Solid state light emitting devices, including solid state lampsincluding LEDs are extremely useful because they potentially offer lowerfabrication costs and long term durability benefits over conventionalincandescent and fluorescent lamps. Due to their long operation (burn)time and low power consumption, solid state light emitting devicesfrequently provide a functional cost benefit, even when their initialcost is greater than that of conventional lamps. However, because largescale semiconductor manufacturing techniques can be used, many solidstate lamps can be produced at extremely low cost. One such device isthe solid state light emitting diode (LED) which has low fabricationcosts, long operational lifetimes and low maintenance costs.

Light emitting diodes (LEDs), and similarly constructed superluminescent diodes and semiconductor diode lasers, are commerciallyavailable and a wide variety of designs and manufacturing techniqueshave been developed. In addition to applications such as indicatorlights on home and consumer appliances, audio visual equipment,telecommunication devices and automotive instrument markings, such LEDshave found considerable application in indoor and outdoor informationaldisplays. But until recently, LEDs have produced light only in the red,green or amber ranges and have not been generally suitable forreplacing, for example, incandescent bulbs, with normally a whiteluminescence, in a wide variety of display applications. The recentintroduction of a bright blue LED, however, allows white light LEDsystems to be realized and thus has the potential to open the displaymarket to LEDs by providing a practical means to achieve both full colorand white light illumination.

The practical advantages of LED displays over those using incandescentbulbs are many. The operational lifetime (in this case, defined ascontinual illumination) of a LED is on the order of ten years or over50,000 hours, whereas incandescent bulbs often burn out in the order of2000 hours, thus leaving an empty pixel in the display message. Suchrecurrent failures make a display unreadable and, therefore, not useful.These conditions (i.e., broken or missing pixels) require constantrepair leading to a significant maintenance problem for providers ofdisplay signs based on incandescent illumination devices. With the longoperational lifetime of a LED-based sign board, the pixels rarely burnout and the illuminated message remains legible over long operationalperiods.

Similarly, LED lamps are considerably more robust. When exposed tostress, mechanical shocks, or temperature variations often encounteredin an outdoor environment they are less likely to fail than incandescentlamps. This attribute is especially important when the signage isutilized in an environment such as vehicular traffic, e.g., roadwaysignage to mark highway construction sites, bridges, tunnels, or trafficcontrol markings, in which perishable filaments used in the incandescentlamps frequently break due to constant vibrational motion. Further,incandescent and fluorescent lamps are constructed with fragile glassexterior casings whose breakage makes the lamp useless, and byextension, the message on the sign board illegible. Due to severeenvironmental conditions on roadways, glass breakage of incandescent andfluorescent lamps is an all too frequent mishap. The solid state LEDlamp has no filaments to break and is housed within a durable plasticcasing, as the primary device envelope or package (typically being ofconsiderable thickness), thereby exhibiting a high level ofimperviousness to extreme outdoor environmental stresses. With respectto outdoor signage applications, displays can contain up to 1 million ormore pixels or lamps. Thus the maintenance costs related to replacementof non-operational incandescent lamps or miniature fluorescent (or neon)lamps are high and unfortunately, continual.

Hence, an emerging trend in the manufacturing and marketing ofinformational displays or signage, especially for outdoor usage, is toutilize solid state LED lamps as replacement for more conventionalincandescent bulbs. The major end user benefits are the lower powerconsumption costs and the longer operational lifetime (hence, reducingmaintenance costs). A further benefit is the rapid relaxation times of asolid state device affording an opportunity to display rapidly changinginformation messages incorporating video or lifelike animation.

Given the desirability of white light displays (e.g., commercial bank“time and temperature” message boards, stadium scoreboards),considerable effort has been expended to produce white light LEDs.Although the recent availability of the blue LED makes a full color, andby extension a white light display realizable, conventionally it hasbeen considered that such a display would require multiple LEDs. Themultiple LEDs would be then incorporated into complicated and expensiveLED modules to obtain the required broad band illumination necessary toprovide white light. Even if a discrete LED lamp were constructed thatprovides white illumination (as opposed to the utilization of amultitude of single die, single color discrete LED lamps in a module orsub-assembly), the current state of the art requires the utilization ofmultiple LED dies and typically at least four electrical leads to powerthese dies. U.S. Pat. No. 4,992,704 issued to Stinson teaches a variablecolor light emitting diode having a unitary housing of clear moldedsolid epoxy supporting three LED dies characterized as producing colorhues of red, green and blue, respectively. There have been some recentintroductions of commercial “full-color” LED lamps, that are essentiallydiscrete lamps which afford a means of producing white light. Allcurrently available examples of such lamps contain a minimum of threeLED dies (or chips)- one red, one green and one blue, encapsulated in asingle epoxy package. The chips are powered via at least 4 electricalleads. These complicated multiple die, variable color devices provide anexpensive and complicated method of offering white light illumination.Furthermore, these multiple die white lamps are rather inefficient inthe present state of the art, offering luminosity far below thatrealized by existing monochromatic light emitting diode lamps, even whena very large quantity of dies are functionally incorporated into thediscrete lamp assembly.

The utility of solid state lamps that offer white light illumination isclear. However, at present there is a very limited number of such solidstate lamps available. In signage applications where a small pixel oflight is frequently required to offer the highest possible resolution ofthe message or video image, the most practical solid state lamps fordisplay applications are the LED lamps. The LED lamp can have verynarrow angles of irradiance and are very small in size when comparedwith other means of providing a radiant surface. However, the methods offabricating white LED lamps are limited. A conventional approach is tofabricate a large cluster of red, green and blue LED discrete lamps,housed in multiple lamp (up to 30) subassemblies or modules. Byproviding multiple power sources to control all of the discrete lamps,these large modules can appear, from a distance, to provide white lightby the spatial mixing of blue, green and red sub-pixels of light givenoff by the individual discrete LED lamps that comprise the module. Whilethe lamps that make up the modules may be individually addressable, andhence, offer the opportunity to, selectively and individually, providered, green and blue light (or combinations thereof), such modularsystems are complex and costly means of providing white light for asolid state display. Further, as these modules are rather large, theultimate resolution of the display will always be lower than that of aconventional single lamp pixel display.

Whereas multiple discrete LED dies housed within a single polymericmatrix (as taught by Stinson) may provide a discrete LED lamp such thatthe illumination could appear white to an observer, the individual LEDdies would still need to be individually powered and the lamp wouldrequire multiple leads in order to effect the simultaneous emission ofmultiple wavelength light. Thus, this multiple die LED lamp would berather expensive to fabricate, and would require expensive andcomplicated circuitry to power and control in an outdoor display.Despite these problems, both methods point to the utility of generatingwhite illuminance.

It would thus be highly desirable to develop a simple solid state LEDlamp, with a minimum of power leads, (i.e., 2) exactly as practiced insingle color LED lamps, such that three domains of red, green and bluelight are generated and yet the white light emission is apparent to anobserver, all while offering significantly reduced die costs (one versusthree) and low fabrication costs in the design of corresponding displaysand signage, high medium resolution (small pixel or lamp size), rapidswitching to the on and off states (to enhance live video imaging), andwith a high luminous efficiency.

It is well known that so-called fluorescent lamps provide white lightillumination. In a fluorescent lamp, the Hg vapor in the vacuum tube isexcited by an electrical discharge. The excited Hg atoms emit light,primarily in the ultraviolet region (e.g., 254 nm, 313 nm, 354 nm),which is absorbed by the inorganic phosphors coating the inside walls ofthe tube. The phosphors then emit light. These inorganic phosphors aredesigned as such to offer white light emission by “down-converting”(i.e., transforming a higher frequency, shorter wavelength form ofenergy to a lower frequency, longer wavelength form of energy) theultraviolet emissions of the excited states of atomic Hg into a broadspectrum of emitted light which appears as white to the observer.However, these light emitting devices are not solid-state, andminiaturization of these fluorescent bulbs to provide suitable pixelresolution for display applications has never been practicallyaccomplished. In fact, the primary application of miniature fluorescentlamps (with long operational lifetimes but unfortunately high powerconsumption when compared with solid state LED lamps) in displays is toprovide back lighting to liquid crystals that are individually addressedat the pixel level. Furthermore, these miniature fluorescent lampsremain fragile light emitting devices by virtue of their glass housingsand are unsuitable for use in display applications in which the lampsare exposed to extreme environmental stresses. Such stresses can notonly break the glass housing, but effect delamination of the powdercoatings from the interior wall of the glass housing. It would bedesirable to generate white light by radiative energy transfer, wherethe luminescent centers are an integral part of the assembly such that athick, difficult-to-fracture housing structure (plate or bulb) couldprovide white illumination from the interior thickness of such housingstructure, and not from a semi-permanent powder coating placed on oneside of a housing surface.

In a further example of generating white light, in the absence ofphosphor coatings, it was disclosed in Chao, et al., “White LightEmitting Glasses,” Journal of Solid State Chemistry 93, 17-29 (1991)(see also El Jouhari, N., et al., “White light generation usingfluorescent glasses activated by Ce³⁺, Tb³⁺ and Mn²⁺ ions,” Journal dePhysique IV, Colloque C2, supplement au Journal de Physique III, Volume2, October 1992, C2-257 to C2-260), that vitreous materials are capableof generating white light by simultaneous emission of blue, green andred emitting fluorescent centers in B₂O₃-based glass that simultaneouslycontain Ce³⁺, Tb³⁺, and Mn²⁺ as activators. These glasses provide whiteillumination by offering the blue emission of Ce³⁺ as well as by thetransfer of excited state energy from the Ce³⁺ to Te³⁺ and Mn²⁺, whoseluminescence occurs respectively in the green and red parts of thevisible light spectrum.

Mixed rare earth borates can be used to provide white lightillumination, via down conversion, with excitation of the borate powderswith a primary (ultraviolet) radiation between 250 nm and 300 nm.Similarly, for cathode ray applications, white light-emitting mixedfluorescent materials can be made by careful formulation of greenfluorescent materials (48 to 53% w/w), red fluorescent materials (37 to40% w/w) and blue fluorescent materials (10 to 13% w/w).

While the devices in the above examples vary in concept andconstruction, they demonstrate the utilization of red, green and bluefluorescent materials, all inorganic in composition which when excitedby photons or electron beams, can release multiple wavelengths ofsecondary light emission (luminescence of either fluorescent orphosphorescent character) to exhibit white light to the observer. Thisis generally true, even if microscopic domains of discrete colored lightemission can be observed on the Lambertian surface of the light emittingdevice.

Tanaka, S., et al., “Bright white-light electroluminescence based onnonradiative energy transfer in Ce- and Eu-doped SrS films,” App. Phys.Lett. 51 (21), Nov. 23 1987, 1662-1663, describes the generation of awhite-light emitting thin-film electroluminescent (EL) device using Ce-and Eu-doped strontium sulfide (SrS) inorganic phosphors. In the ELexcitation of the SrS:Ce,Eu device, nonradiative energy transfer fromthe Ce³⁺ luminescent center to the Eu²⁺ luminescent center plays animportant role in generating broad EL emission extending from the blueto the red, thereby generating white light.

Similarly, some recent discussions of AlGaN electroluminescent systemswith Zn and Si dopants have indicated that some white light can begenerated. While it is useful for a single device to be constructed inwhich dopants offer a multitude of luminescent wavelengths, dopantsinvariably alter the electrical and lattice structures of semiconductorsand as such, the performance of these devices are considerably poorerthan for corresponding semiconductors free of dopant that emitmonochromatic irradiation, as a result of being dopant-free.

Until recently, most light emitting diodes have been semiconductor-basedand most electroluminescent devices have been inorganic based. Whileorganic materials have been utilized to prepare certain thin-filmelectroluminescent devices, no organic based LEDs are commerciallyavailable. Further, organic-based LEDs are at present plagued byextremely short operational lifetimes due to degradation of the organiccharge-transfer materials. In all of these systems, the organicmaterials, used in thin films on conducting inorganic substrates such asITO, are actively participating in the electron hole recombinationnecessary to generate an excited state, and, by subsequent radiativedecay, light.

Recently, the literature has discussed approaches directed tofabricating organic LED or electroluminescent devices and in certaincases, white light emission has been observed from these experimentaldesigns. As an example, white light from an electroluminescent diodemade from poly[3(4-octylphenyl)-2,2′-bithiophene] and an oxadiazolederivative have been reported. Spectroscopic analysis indicates that theapparent white light is composed of blue (410 nm), green (530 nm), andred-orange (520 nm) luminescent centers. Electroluminescent devicesincorporating the red fluorescing material Rhodamine onto an inorganicsubstrate have been effective in yielding some white light as well.

White light emission from thin film organic electroluminescent cellsbased on poly(vinylcarbazole PVK) thin films on ITO-coated glass hasalso been recently reported. The cell has the construction ofMg:Ag:Alq:TAZ:doped PVK:ITO:Glass where the conducting ITO layer injectsholes into the organic based PVK thin film layer which has high holedrift mobilities. Simultaneously, electrons are injected by thetris(8-quinolato) aluminum (III) complex layer Alq, into the holeblocking electron transporting layer composed of the organic molecule3-(4′tert-butylphenyl)-4-phenyl-5-(4′-biphenyl)-1,2,4-triazole, TAZ. Atthe interface of the organic poly(vinlycarbazole) layer with the TAZlayer, recombination of holes and electrons take place which excites theorganic, aromatic, carbazole pendant moiety that comprises the polymer.It is well known that the excited carbazole moiety within the polymeraggregates in the excited state leads to blue excimer emission, in theabsence of quenchers or dopants. In the example of the organicMg:Ag:Alq:TAZ:doped PVK:ITO:Glass electroluminescent device, thequenchers of excimeric emission, are the dopants blue emitting1,1,4,4-tetraphenylbuta-1,3-diene (TPB), green emitting7-diethylamino-3-(2′benzothiazoyl)coumarin (Coumarin-6), and redemitting dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran(DCM-1).

U.S. Pat. No. 5,045,709 issued Apr. 11, 1995 to J. E. Littman et al.discloses a white light emitting internal junction organicelectroluminescent device comprising an anode, an organicelectroluminescent medium and a cathode. The organic electroluminescentmedium further comprises a hole injecting and transporting zonecontiguous with the anode, and an electron injecting and transportingzone contiguous with the cathode. The electron injecting andtransporting zone further comprises an electron injecting layer incontact with the cathode. The portion of the organic electroluminescentmedium between the electron injecting layer and the hole injecting andtransporting zone emits white light in response to the hole-electronrecombination, and comprises a fluorescent material and a mixed ligandaluminum chelate.

Japanese Patent Publication 04289691 of Mitsubishi Cable Industries,Ltd., published Oct. 14, 1992, discloses an electroluminescent devicecomprising a fluorescent dye-fixed silica layer coated with atransparent electrode layer, a luminescing (light-emitting) layercontaining a phosphor, a backside electrode layer, a water-sorbinglayer, an encapsulating film, and an insulating layer.

In the Mitsubishi patent publication, the silica layer may be formed bya so] gel process using metal alkoxides in a solvent such as ethanol,isopropanol, or dimethyl ether. A Rhodamine 6G-doped silica layer isdescribed to exhibit white luminescence. The luminescing layer may befor example on the order of 15 microns in thickness, and is formed by asol gel technique yielding ZnS or ZnCdS doped with a dopant such ascopper, aluminum, manganese, chlorine, boron, yttrium, or rare earthdopant. The luminescing layer may also contain scattered phosphormaterial. The average grain size of grains in the luminescing layer isgenerally greater than 10 microns, and preferably is in the range offrom 15 to 40 microns. The luminescing layer may for example containfrom 30 to 80% phosphor. A disclosed advantage of the foregoingstructure is that one can change the phosphor in the luminescing layer,and thereby change the color of the whole material.

Japanese Patent Publication 60170194 of Sony Corporation, published Sep.3, 1985, discloses a white light-emitting electroluminescent device witha luminescent layer containing a mixture of a blue-green-emittingphosphor and Rhodamine S. Since Rhodamine S strongly fluoresces orangeby excitation with a bluish-green light, a white light of highluminosity may be obtained even at low voltage. This reference disclosesa phosphor emitting blue-green light, in which ZnS is doped with Cu andCl, as well as a phosphor emitting yellow light, in which ZnS is dopedwith Cu and Mn. ZnS may also be doped with Cu and Br to produce greenlight.

The Sony patent publication discloses a multilayer electroluminescentarticle, including sealing layers of protective film of a material suchas Aclar polymer, a polyester layer, a transparent electrode formed ofindium tin oxide (ITO), a light-emitting layer, and a backsideelectrode. The light-emitting layer may comprise 50-95% by weight of ZnSdoped with the aforementioned dopant species (e.g., 0.045% wt. Cu, and0.020% wt. Cl) and 5-50% wt. Rhodamine S.

Not withstanding the progress made in using organic fluorescers asluminescent sites within either electron-transport or hole-transportlayers and affording thin-film interfacial hole-electron recombination,the current state of the art finds it difficult to generate organicbased LED dies with reasonable operational lifetimes. By their verynature, these donor-acceptor complexes are prone to reaction with thesurrounding medium. As a result, many of these organic molecules degradeunder constant excitation to the excited state and consequently theorganic-based LEDs fail. Those fluorescers with extremely high quantumyields of fluorescence, which by definition necessitate short excitedstate lifetimes and are unlikely to be quenched or degraded by oxygen orother reactants, do not have sufficient electron or hole transportproperties to allow for device-wide localized hole-electronrecombination in the ground state. However, their proximity to theholes, as dopants in a hole transporting layer, as an example, may makethe excited states of the luminophors more easily oxidized than wouldnormally be the case. This would be especially true for excited statespecies, even if the ground state of the luminophors are stable to theholes in the hole-transporting layer. Similarly arguments regardingexcited state reduction would be applicable for dopants sequesteredwithin an electron-transport layer.

It would be most desirable, then, if a white light emitting LED devicecould be fabricated that took advantage of the simultaneous emission ofred, green and blue luminescent centers, using both inorganic andorganic fluorescers or phosphors without requiring these species to bein proximate contact with the transporting layers.

It is the purpose of the present invention to provide white light solidstate luminescent devices using a single die, which initially providemonochromatic radiation and wherein the monochromatic radiation isconverted to polychromatic white light, thus providing a solid stateillumination device with white illuminance, without the need formultiple power leads or for more than one discrete LED lamp.

SUMMARY OF THE INVENTION

The present invention relates broadly to a light emitting assemblycomprising a solid state device which is suitably joined by circuitforming means to a power supply, constructed and arranged to power thesolid state device and induce the emission from the solid state deviceof a first, relatively shorter wavelength radiation. The solid statedevice is structurally associated with a recipient down-convertingluminophoric medium which when impinged by the first, relatively shorterwavelength radiation is excited to responsively emit a radiation in thevisible white light spectrum.

In accordance with a specific embodiment of the present invention, anLED operative to emit, for example, monochromatic blue or ultraviolet(UV) radiation is packaged along with fluorescent organic and/orinorganic fluorescers and phosphors in an insulating polymeric matrix.The monochromatic blue or UV radiation output of the LED is absorbed andthen down converted by the fluorphore or phosphor to yield longerwavelengths to include a broad spectrum of frequencies which appear aswhite light.

This use of fluorescers and/or phosphors to effect down conversion oflight from an LED in a solid state light emitting device using a packingdye material is a significant departure from prior art teaching. Inaddition to allowing for the generation of white light from a blue orultraviolet emitting LED die with a typical p-n junction construction,devices in accordance with the invention can be variously constructed toprovide an essentially infinite series of colored (visible) lightemissions, of either narrow or broad spectral distribution; from onesingle p-n junction construction. The concept can be extended to anysolid-state light emitting device, including super luminescent diodes,diode layers, electroluminescent cells, electroluminescent displays,organic and polymer based light emitting diodes and/or devices, eventhose not requiring semiconductor p-n junctions, providing an insulatingmatrix or housing can be attached to or incorporated within the device.

As used herein, the term “solid state device,” used in reference to thedevice for generating the primary radiation which subsequently isdown-converted to a longer wavelength radiation in such visible whitelight spectrum, means a device which is selected from the groupconsisting of semiconductor light emitting diodes, semiconductor lasers,thin film electroluminescent cells, electroluminescent display panels,organic based light-emitting diodes, polymeric-based light-emittingdiodes, and internal junction organic electroluminescent devices.

As used herein, the term “luminophoric medium” refers to a materialwhich in response to radiation emitted by the solid state device emitslight in the white visible light spectrum by fluorescence and/orphosphorescence.

Other aspects, features and embodiments of the invention will be morefully apparent from the ensuing description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic elevational cross-sectional view of adown-converting solid state device assembly for producing white lightaccording to one embodiment of the present invention.

FIG. 2 is a schematic elevational cross-sectional view of another whitelight generating assembly according to another embodiment of theinvention.

FIG. 3 is a schematic elevational cross-sectional view, in enlargedscale, of a portion of the device of FIG. 1.

FIG. 4 is a schematic representation of a display which may usefullyemploy the device of FIGS. 1 and/or 2.

FIG. 5 is a schematic elevational view of an electroluminescent celldevice according to another embodiment of the invention.

FIG. 6 is a schematic representation of the generalized light emittingassembly of the present invention.

DETAILED DESCRIPTION OF THE INVENTION, AND PREFERRED EMBODIMENTS THEREOF

The present invention is based on the discovery that a highly efficientwhite light-emitting device may be simply and economically fabricatedutilizing a solid state light emitting device for generating a shorterwavelength radiation which is transmitted to a luminophor (fluorescentand/or phosphorescent solid material) for down conversion by theluminophor of the radiation from the solid state light emitting device,to yield white light.

White light LED solid state devices may be made by the method of thepresent invention, utilizing a down conversion process whereby theprimary photon generated in the active region of the diode is downconverted with primary blue emission and/or secondary blue fluorescentor phosphorescent centers, as well as green and red fluorescent orphosphorescent centers. Such an LED device is able to down-convert therelatively monochromatic light, typical of all heretofore colored LEDdies and lamps, to a broader emission that provides white light fromred, green, and blue emission centers. Such a device for white lightemission, based on down-conversion, requires the primary light to beeither blue or ultraviolet emission, such as is available using blue orultraviolet LED dies and lamps. It is an important element of thisconsideration that both inorganic and organic fluorescent orphosphorescent materials can be utilized to down-convert the primaryultraviolet or blue light emission to a mixture of blue, green and redluminescent emissions. A significant advantage of organic luminescentmaterials is their relatively broad emission bandwidth which offers themaximal overlap of photon wavelengths to most readily generate a whiteillumination. Further, it is most desirable to utilize organicfluorescent materials with extremely short radiative lifetimes, lessthan 50 nanoseconds, to preclude non-radiative energy transfer (to thelowest energy emitter).

As discussed above, there have been disclosures regarding the generationof white light in solid state illumination devices using radiative ornon-radiative energy transfer and these examples use primarily inorganicdopants in the active layers of electroluminescent cells or displaysystems, but none are known that apply the principles of the presentinvention to semiconductor based p-n junction LED lamps.

Referring now to the drawings, FIG. 1 shows a white light emitting diodeassembly 10 constructed in accordance with the invention. This assemblycomprises an enclosing wall 7 defining a light-transmissive enclosure 11having an interior volume therewithin. The enclosure 11 may be formed ofany suitable material having a light-transmissive character, such as aclear or translucent polymer, or a glass material. Thelight-transmissive enclosure 11 houses in its interior volume a lightemitting diode (LED) die 13 positioned on support 14. First and secondelectrical conductors 16 and 17 are connected to the emitting and therear faces 18 and 19 of LED die 13, respectively, and with the emittingface 18 of the LED die coupled to the first electrical conductor 16 bylead 12. The enclosure is filled with a suitable down-convertingmaterial 20, e.g., a down-converting medium comprising fluorescer and/orphosphor component(s), or mixtures thereof, viz., a luminophoric medium,which functions to down convert the light output from face 18 of LED 13to white light.

In one embodiment, LED 13 comprises a leaded, gallium nitride based LEDwhich exhibits blue light emission: with an emission maximum atapproximately 450 nm with a FWHM of approximately 65 nm. Such a deviceis available commercially from Toyoda Gosei Co. Ltd. (Nishikasugai,Japan; see U.S. Pat. No. 5,369,289) or as Nichia Product No. NLPB520,NLPB300, etc. from Nichia Chemical Industries, Ltd. (Shin-NihonkaikanBldg. 3-7-18, Tokyo, 0108 Japan; see Japanese Patent Application4-321,280). The down-converting material in this embodiment comprises ablue fluorescer (Lumogen® F Violet 570-substitutednapthalenetetracarboxylic diimide), a green-yellow fluorescer (Lumogen®F Yellow 083-substituted perylenetetracarboxylic diimide) and a redfluorescer (Lumogen® F Red 300-substituted perylenetetracarboxylicdiimide). A composition comprising such blue, green-yellow, and redfluorescent materials, all organic based, as incorporated in aninsulating epoxy polymer, is available commercially from PacificPolytech (Pacific Polytech, Incorporated, 15 Commercial Blvd., Novato,Calif. 94949-6135).

Both gallium nitride and silicon carbide LEDs are suitable forgenerating light at appropriate wavelengths and of sufficiently highenergy and spectral overlap with absorption curves of thedown-converting medium. The LED preferably is selected to emit mostefficiently in regions where luminescent dyes may be usefully employedto absorb wavelengths compatible with readily commercially availablefluorescers and/or phosphors for down conversion to white light.

The luminophoric medium utilized in the light emitting assembly of thepresent invention thus comprises a down-converting material which mayinclude suitable luminescent dyes which absorb the radiation emitted bythe LED or other solid state device generating the primary radiation, tothereby transfer the radiation energy to the fluorescer(s) and/orphosphor(s) for emission of white light. Alternatively, the luminophoricmedium may comprise simply the fluorescer(s) and/or phosphor(s), withoutany associated mediating material such as intermediate luminescent dyes,if the fluorescer(s) and/or phosphor(s) are directly excitable to emitthe desired white light.

Such a light emitting assembly is shown in FIG. 2, wherein the samegeneral structure is shown as in FIG. 1 (with the same referencenumerals of corresponding parts for ease of reference), but in place ofthe luminophoric medium 20 shown in the embodiment illustrated in FIG.1, the assembly of FIG. 2 utlilizes a fluorescer associated with thelight-transmissive housing 11. The fluorescer in such embodiment may beeither dispersed in the wall 7 of the housing structure, and/or coatedas an interior film 9 of the fluorescer on the interior wall surface ofthe housing wall 7. Alternatively, the fluorescer may be coated on anexterior wall surface of the housing of the assembly (not shown), if thehousing is ultimately deployed in an environment where such exteriorcoating may be satisfactorily maintained in an operable state (e.g.,where it is not subject to abrasion, or degradation). The fluorescermaterial may for example be dispersed in the polymer or glass melt fromwhich the housing subsequently is formed, so as to provide a homogeneouscomposition of the housing wall providing light output from the entirearea of the housing.

Comparing the structures of the FIGS. 1 and 2 assemblies, it is seenthat the luminophoric medium in the FIG. 1 embodiment is contiguouslyarranged about the LED die structure in the interior volume of thehousing, while the luminophoric medium in the FIG. 2 embodiment isdisposed in spaced relationship to the LED die structure. It will beapparent that the specific arrangement of the solid state device such asLED 13, relative to the down-converting medium of the assembly, may bewidely varied in the broad practice of the invention, it being necessaryonly that the solid state device functioning as the source of theprimary shorter wavelength radiation be in transmitting relationship tothe recipient luminophoric medium, so that the latter functions in useto down-convert the transmitted radiation from the solid state deviceand responsively thereto emit white light.

An ultraviolet LED light source suitable for use in the structure ofFIG. 1 may comprise:

aluminum gallium indium nitride; aluminum gallium nitride; indiumgallium nitride; gallium nitride or any other ultraviolet emittingdiode. A blue LED light source may be based on:

indium gallium nitride; silicon carbide; zinc selenide; or any otherblue light emitting diode source.

TBP, Coumarin-6 and DCM-1, as described by Kido et al. in EuropeanPatent EP 647694, are suitable materials for down conversion of theoutput of gallium nitride or silicon carbide LEDs. Gallium nitride andits alloys can emit in the spectral range covering the blue andultraviolet extending from wavelengths of 200 nanometers toapproximately 650 nanometers. Silicon carbide LEDs emit most efficientlyin the blue at wavelengths of around 470 nanometers.

If gallium nitride emitters are employed, preferred substrates for theemitters include silicon carbide, sapphire, gallium nitride and galliumaluminum indium nitride alloys, and gallium nitride-silicon carbidealloys, for achieving, a proper lattice match.

With ultraviolet or blue light LEDs, aromatic fluorescers may beemployed as down-converting emitters. By way of example, suitablefluorescers could be selected from:

A) blue luminescent compositions-9,10-diphenylanthracene;1-chloro-9,10-diphenylanthracene; 2-chloro-9,10-diphenylanthracene;2-methoxy-9,10-diphenylanthracene; 1,1,4,4-tetraphenyl-1,3-butadience(TPB), Lumogen® F Violet 570 (a substituted napthalenetetracarboxylicdiimide); Alq₂OPh (were Al is aluminum, q is 8-hydroxyquinolate, and Phis phenyl);

B) green-yellow luminescent compositions-9,10-bis(phenylethynyl)anthracence; 2-chloro-9,10-bis(phenylethynyl)-anthracene;Coumarin-5(7-diethylamino-3-(2′benzothiazoyl)coumrin); Lumogen® Yellow083 (a substituted perylenetetracarboxylic diimide); and Mq₃ (where M isa Group III metal, such as Al, Ga or In, and q is 8-hydroxyquinolate);and

C) red-orange luminescent materials-DCM-1; Lumogen® F Red 300 (asubstituted perylenetetracarboxylic diimide); Lumogen® F Orange 240 (asubstituted perylenetetracarboxylic dimide); tetraphenylnapthacence;zinc phthalocyanine; [benzoythiazoylidene)methyl]- squaraines;tris(bipyridine-ruthenium (2+); and [3]-catenand complexes with copper.

The amount of dyes or fluorescers specifically formulated into theluminophoric medium, which may for example include a polymeric matrix orother matrix material in which the dyes and/or fluorescers are solubleor dispersable, is not specifically limited, and suitable amount(s) ofsuitable material(s) for such purpose can be readily determined withoutundue experimentation, to provide good white light emission (ofvirtually any tint or hue), as well as a virtually infinite series ofchromaticity for all visible hues.

The concentrations of the fluorescers may suitably be determined by boththeir luminescence quantum yields and spectral distribution, as requiredto define a particular color by its respective chromaticity coordinates,as well as, in the case of radiative energy transfer (but not Forsterenergy transfer), the absorption extinction coefficients of theassociated fluorescer(s). Such fluorescers may for example be blue lightfluorescers used with a blue-emitting semiconductor-based LED die, orultraviolet light fluorescers used with a UV-emittingsemiconductor-based LED die. While the concentrations of the variousdyes may be suitably adjusted to realize the required colors, the rangeof dye concentrations typically will be between 10⁻³ to 10 mole per centfor each individual fluorescent component.

FIG. 3 shows the LED structure of LED die 13 of FIG. 1 in an enlargedscale, as comprising substrate 21 mounted on support 14, and coupled inconducting relationship with second electrical conductor 17. Thesubstrate comprises a surface layer 22 formed in accordance with wellunderstood techniques. Referring back to FIGS. 1 and 2, the lightemitted from LED 13 is absorbed by a down-converting dye in theluminophoric medium 20 contained within enclosure 11 (FIG. 1), or by adown-converting dye in the interior film 9 on the interior wall surfaceof housing wall 7 (FIG. 2), to responsively produce white (or fullcolor) light.

FIG. 4 illustrates the use of white light emitting diode deviceassemblies 11 of a type as shown in FIGS. 1 and 2, arranged in an arraycomprising a regular pattern of such assemblies, as components of adisplay 30, or alternatively for a back light illumination panel for astructure such as a liquid crystal display. The individual assemblies 11may be selectively illuminated, by imposing a desired turn-on voltageacross the first and second electrical conductors 16 and 17 (not shownin FIG. 4; see FIGS. 1 and 2), to display a message or design in amanner well understood in the art.

The selective illumination of the component light emitting assemblies 11of the FIG. 4 display is suitably controlled by a controller 31 inresponse to user input. The individual light emitting assemblies 10 ofFIGS. 1 and 2 are connected electrically with suitable electricalcircuitry (not shown) in display 30, in a manner analogous to that usedfor displays utilizing flurorescent or incandescent lamps.Alternatively, all of the component light emitting assemblies 10 may beilluminated simultaneously for back lighting applications.

The light-emitting assemblies shown in FIGS. 1 and 2 may be made in anysuitable size and dimensional character. In application to displays,such light-emitting assemblies will generally be of a size commensuratewith the size of fluorescent or incandescent lamps used in similardisplays.

FIG. 5 is a schematic elevational view of an electroluminescent cellapparatus 40 according to another embodiment of the invention. Thisapparatus 40 comprises end wall members 48, top wall member 49, andbottom wall member 47, which cooperatively with front and rear wallmembers (not shown for clarity), form an enclosure defining interiorvolume 61 therewithin. The top wall member 49 is formed of alight-transmissive material of construction.

The interior volume 61 of the electroluminescent cell apparatus 40contains a white light-emitting polymer 63 which is responsive todown-convert the radiation produced by the LED array in the interiorvolume. The LED array comprises a conductive substrate 42 of a suitablematerial on which are arranged a mulitplicity of LED dies 41, each inelectrical contact at its bottom face with the substrate 42. Thesubstrate 42 in turn is joined to a lead 44 which passes exteriorly ofthe cell apparatus via a feedthrough in bottom wall member 47, and isjoined in circuit-forming relationship to a suitable power supply means(not shown). The LED dies 41 at their top faces are joined in serieswith one another by connection wires 43.

The top contact of the LEDs, joined by connecting wires 43, areelectrically coupled by means of electrode 46 to the lead 45 which alsopasses exteriorly of the cell apparatus via a feedthrough in bottom wallmember 47, for joining to the aforementioned power supply also joined tolead 44. Lead 45 is electrically isolated from lead 44.

In operation, the electrical energization of the LED die arraycomprising LED dies 41 effects radiation emission at a first relativelyshorter wavelength which in transmission to the contiguously arrangedlight-emitting polymer 63 causes the polymer to responsively emit whitelight at a second relatively longer wavelength in the visible whitelight spectrum.

FIG. 6 is a schematic representation of a generalized light emittingassembly 80 according to the present invention. In such assembly, theprimary radiation generating device 82, comprising a solid state devicewhich may include one or more, singly or in combinations of differentdevices, of the devices of the group consisting of semiconductor lightemitting diodes, semiconductor lasers, thin film electroluminescentcells, electroluminescent display panels, organic based light-emittingdiodes, polymeric-based light-emitting diodes, and internal junctionorganic electroluminescent devices. Preferably, the solid state deviceis selected from the group consisting of semiconductor light emittingdiodes and semiconductor lasers, and most preferably the solid statedevice is a semiconductor light emitting diode.

The solid state light radiation emitting device 82 as shown is suitablyjoined by circuit-forming wires or leads 84 and 86 to a power supply 88,constructed and arranged to power the solid state device and induce theemission from the solid state device 82 of shorter wavelength radiation94, preferably in the wavelength range of blue to ultraviolet. The solidstate device 82 is structurally associated with a recipientdown-converting luminophoric medium 90 (the structural association beingschematically represented in FIG. 6 by the dashed line 92, and which maytake the form of a contiguous relationship in a conjoint or unitarystructure, or a spaced relationship therebetween in a same structure, asfor example is shown in the illustrative embodiment of FIG. 2 herein).

The luminophoric medium 90 when impinged by the radiation 94 of ashorter wavelength, is excited to responsively emit a radiation 96having a wavelength in the visible light spectrum. The radiation 96 maybe emitted in a range of wavelengths which combine to produce lightperceived as white.

It will be apparent from the foregoing that the light-emitting assemblyof the present invention may be variously configured with a number ofsolid state light-emitting devices, which emit shorter wavelengthradiation, and transmit such radiation to a luminophoric medium whichdown-converts the applied radiation to yield a white light emission fromthe luminophoric medium.

Further, while the invention has been described primarily herein inreference to the generation of white light, it will be apparent that thescope of the invention is not thus limited, but rathers extends to andencompasses the production of light of other colors than mixed whitelight, utilizing solid state primary radiation emitters, anddown-converting luminophoric media.

Thus, while the invention has been described with reference to variousillustrative embodiments, features, aspects, and modifications, it willbe apparent that the invention may be widely varied in its constructionand mode of operation, within the spirit and scope of the invention ashereinafter claimed.

1. A light emission assembly, comprising at least one light emissiondevice including an LED energizable to emit radiation in the blue toultraviolet spectrum, and a luminophoric medium arranged to be impingedby radiation from the LED in the blue to ultraviolet spectrum and toresponsively emit radiation in a range of wavelengths, so that radiationis emitted from the light emission device as a white light output. 2.The light emission assembly of claim 1, comprising a circuit including apower supply coupled to said at least one light emission device.
 3. Thelight emission assembly of claim 1, comprising a multiplicity of saidlight emission devices.
 4. The light emission assembly of claim 3,wherein said light emission devices are arranged in an array.
 5. Thelight emission assembly of claim 3, comprising a display in which eachof said light emission devices constitutes a pixel element of thedisplay.
 6. The light emission assembly of claim 5, wherein each of saidlight emission devices is adapted to be selectively illuminated so thatthe display produces messages.
 7. The light emission assembly of claim5, wherein each of said light emission devices is adapted to beselectively illuminated so that the display produces designs.
 8. Thelight emission assembly of claim 3, wherein each of said light emissiondevices are connected to electrical circuitry.
 9. The light emissionassembly of claim 8, further comprising a controller adapted toselectively illuminate individual ones of the multiplicity of said lightemission devices, in response to input to the controller.
 10. The lightemission assembly of claim 3, comprising a back lighting assemblyincluding said multiplicity of said light emission devices.
 11. Thelight emission assembly of claim 10, further comprising a displayadapted to be back lit by said back lighting assembly.
 12. The lightemission assembly of claim 11, wherein said display comprises a liquidcrystal display.
 13. The light emission assembly of claim 1, comprisingan enclosure in which said at least one light emission device isdisposed.
 14. The light emission assembly of claim 13, wherein saidenclosure includes a light-transmissive structure.
 15. The lightemission assembly of claim 13, including a multiplicity of said LEDs.16. The light emission assembly of claim 15, wherein said LEDs compriseLEDs mounted on a conductive substrate in said enclosure, in seriesrelationship to one another.
 17. The light emission assembly of claim 1,wherein the luminophoric medium comprises phosphor material.
 18. Thelight emission assembly of claim 1, wherein the luminophoric mediumcomprises a material responsively emitting radiation in the green toyellow spectrum.
 19. The light emission assembly of claim 1, whereinsaid LED comprises a blue light LED.
 20. The light emission assembly ofclaim 1, wherein said LED comprises a GaN LED energizable to emit bluelight having an emission maximum at approximately 450 nm.
 21. The lightemission assembly of claim 1, wherein said white light output comprisesprimary radiation emission from the LED and secondary radiation emissionfrom the luminophoric medium.
 22. The light emission assembly of claim1, wherein said at least one light emission device comprises an LEDincluding a material selected from the group consisting of: galliumnitride; indium gallium nitride; aluminum gallium indium nitride;aluminum gallium nitride; silicon carbide; and zinc selenide.